As the general population ages disease such as cancer contribute more significantly to the health care burden. Finding ways to diagnose and treat cancer more efficiently is critical to both improve quality of life and overall life span. Amng women, breast cancer presents as the most common type of cancer and is often treated surgically or with general chemo- and radiation therapies. Recent work to characterize, on a molecular level, breast cancer has produced targeted therapies that are effective in certain subtypes of cancer. However, because we do not have a full understanding of the molecular pathways controlling and driving these cancers more effective treatments are not currently possible. An important growth regulator of breast cancer is the sex hormone estrogen. Estrogen's effects on breast cancer are to rewire the transcriptional network. Transcription is a complex process that is regulated in part by the non-protein coding 7SK RNA. Estrogen signaling has been shown to impact the cellular levels of HEXIM1, a protein that interacts with 7SK, however the molecular mechanism for how this deregulation of 7SK through HEXIM1 specifically changes the transcriptional output of breast cancer cells is unclear. This proposal aims to further our mechanistic understanding of 7SK's role in estrogen dependent signaling in breast cancer. Structural characterization of 7SK inside living cells is critical to reveal functioal elements in the RNA. I will utilize new chemical tools to evaluate the structure of 7SK in vivo across an estrogen treatment series (Aim 1). Further, employing a genome-wide method to measure 7SK- chromatin interactions I will elucidate the mechanism by which 7SK regulates transcription inside cells (Aim 2). The combination of in vivo structure measurements and chromatin interaction maps of the 7SK snRNA will begin to address, on a global scale, how one snRNA can act to control transcription. Additionally, the estrogen- dependent changes I measure will be informative for how the transcriptional network is specially affected by hormonal stimulation. The proposed experiments provide clear hypothesis driven experiments for future work in other systems and in an effort to control this pathway for therapeutic use.